Uploading files always null

Help, for some reason i cannot get the uploaded files... It always say "Call to a member function getSize() on null "

This is my controller function

PHP Code:

public function upload_macro(){
helper('form');
helper('file');
$data["msg"]="";
$myfile = $this->request->getFile('myfile');
$myfile->getSize('mb');
return view('home_upload_macro',$data);
} 


My View

PHP Code:

<form action="<?php echo base_url("home")?>/upload_macro" method="post">
     <input type="file" name="myfile" />
     <input type="submit" value="upload"/>
</form>